Full Idea
The trouble with Platonism had always been its inability to define a priori knowledge in a way that made it possible for human beings to have it.
Gist of Idea
Platonism defines the a priori in a way that makes it unknowable
Source
J. Alberto Coffa (The Semantic Tradition from Kant to Carnap [1991], 7 'What')
Book Reference
Coffa,J.Alberto: 'The Semantic Tradition from Kant to Carnap' [CUP 1993], p.126
A Reaction
This is the famous argument of Benacerraf 1973.